This section provides background information related to the present disclosure which is not necessarily prior art.
Direct gas-fired heaters have been manufactured for over 50 years to serve industrial and commercial facilities. In direct fired commercial heaters, circulation air and products of combustion are vented directly into the space being heated, unlike indirect fired heaters that vent combustion products to the outdoors. Direct gas-fired heaters are primarily intended for space heating applications in commercial and industrial facilities to address the heat load and ventilation requirements of these facilities.
The most common use for a direct fired heater is to replace air that is being exhausted as part of a process. When used for this purpose, direct fired heaters are commonly referred to as make-up air heaters. Typically, a make-up air heater is provided to the marketplace in a draw-through heater configuration to meet customer demand for a constant airflow output, e.g., in cubic feet per minute (cfm), that matches the exhaust airflow capacity.
In a draw-through design or configuration, outside air first passes over a line burner before entering a centrifugal blower. The blower is located downstream of the line burner and operates to draw outside air through or over the line burner. The blower in this draw-through arrangement is a “constant volume machine” which indicates that the air volume delivered by the blower to the space is relatively constant as it handles air over a fairly narrow range of outlet temperatures.
Make-up air heaters normally discharge air at or slightly above the desired room temperature. Energy efficiency may be optimized by neutralizing the impact of exhausting air out of a building and replacing it with a heating system that converts all of the sensible heat contained in the gas to heat the outside air that replaces that which was exhausted by the process. The moisture from the combustion process is discharge in the space, which adds to the comfort level of the occupant as the outside air tends to be very dry in the fall and winter months when heat is needed most. Because the moisture derived from the combustion process is not condensed, the overall system efficiency of these types of direct fired make-up air heaters are recognized to be approximately 92% efficient. The inventors hereof have recognized, however, that without the air being provided by the direct fired make-up air heater in a controlled manner, air would enter the building through openings that are part of the building structure in the form of infiltration air. Air that infiltrates a building is untempered air (not preheated), which results in cold drafts and uncomfortable working conditions especially for personnel working in the dock areas where open doors become the pathway of choice for the infiltration air.
Direct gas-fired heaters have also been marketed for over 50 years with a blow-through heater configuration in which the blower is upstream of the burner. More specifically, the blower is located to handle outside air and blow the outside air past a burner, which is operable for heating the outside air before it is discharged into the space to be heated.
But the inventors hereof have recognized that direct fired blow-through heaters are not well suited for exhaust air applications because cold outside air will expand after it passes over the burner as a function of the change in air density. By way of example, air at 0° F. and having an air density of 0.08635 pounds per cubic foot (lbs/ft3) heated to 70° F. after it passes the burner results in an expansion of that air and a lowering of the air density to a value of 0.075 pounds per cubic foot. This, in turn, results in an increase in the airflow being discharged by the heater of 15.1%. If the exhaust application was a kitchen hood in a restaurant, the excess supply air may have a negative impact on the kitchen hoods ability to extract the smoke and odor for the cooking process, which could also impact the remaining public areas of the facilities.
Direct fired blow-through heater configurations are well suited for use as space heaters. In this case, a direct blow-through heater may be applied to address the heat load of a facility and not to match a given exhaust application. Industrial and commercial buildings have an infiltration load element as part of its heat load as a result of wind and temperature differences between indoor and outdoor temperatures. Based on ASHRAE (American Society of Heating, Refrigeration, and Air-Conditioning Engineers) ventilation requirements, it is often necessary to provide a source for this ventilation requirement as well as which can be met by this same heater.
In some well insulated buildings, the infiltration element of the heat load analysis can show that the infiltration load and the load associated with the ventilation requirement is more significant than the conduction load. In these applications, the optimization of a heating system occurs when the system first addresses and matches the combination of infiltration load and ventilation load on a designated day and then checks to verify that the conduction load requirement has also been addressed. When a direct fired heater is utilized for space heating, that portion of the heater's capacity that heats the outside air temperature to room temperature is directly tied to the infiltration and ventilation heat load. That portion of the heater capacity above room temperature and the maximum temperature rise of the heater is applied to the conduction load with any extra capacity also being applied to the any infiltration and ventilation heat load remaining, if required. There is a significant system efficiency advantage if the blow-through heater is capable of obtaining a temperature rise equal to or greater than the maximum discharge temperature allowed by the ANSI (American National Standards Institute) Standard Z83.4 for Non-Recirculating Direct Gas-Fired Industrial Air Heaters. ANSI Standard Z83.4 sets the maximum discharge temperature at 160° F. and limits the maximum temperature rise to 190° F. In an application where the minimum design for a location is 0° F. (e.g., like Saint Louis, Mo., etc.), a heater with a temperature rise of 160° F. would therefore optimize the heater selection for that location.
Another benefit of a direct fired blow-through space heater configuration is that a space heater is generally cycled on and off based on a call for heat by a room thermostat. A conventional draw-through make-up air heater will run continuously as long as the exhaust fan is operating. During the operating time of a space heater, the heater airflow tends to neutralize the flow of infiltration air into the building as a result of the air brought in by the heater escaping out of the same cracks. This exhale of the air supplied by the heater carries out other contaminants that may be created in the building. If the infiltration rate of the building is too low, additional relief openings may be required to meet the minimum ventilation requirements of the facility.
Since the products of combustion in both draw-through heaters and in blow-through heaters are released into the heated air stream, it is important the levels of these combustion products (carbon monoxide (CO), nitrogen dioxide (NO2) and carbon dioxide (CO2)) be controlled by the burner design to meet the levels identified by the ANSI Standard for these products. ANSI Standard Z83.4 covers both configurations of heaters (blow-through and draw-through) and limits the CO rise through the heater to no more than 5.0 parts per million (ppm). In comparison, OSHA (Occupational Safety and Health Administration) indicates the maximum exposure for an 8 hour period for the occupants of a building. If propane fork trucks are utilized in the facility, the combustion products generated by the fork trucks is additive to the environment. And, additional ventilation provided by the operation of these space heaters is required to purge that contamination from the building. It is generally recognized that 5,000 cfm of ventilation air is required per operating and well-tuned fork truck to keep contaminants from approaching undesirable levels.
The NO2 rise through the heater is limited to 0.5 ppm, and CO2 is limited to 4,000 ppm for these heaters by ANSI Standard Z83.4. CO2 production is solely a function of the type of gas utilized (natural gas or propane) and the temperature rise based on a mathematical relationship (CO2=19.63×K×Temp Rise, where K=1.04 for natural gas and 1.206 for propane). NO2 like CO is a function of the burner design and generally limits the maximum temperature rise that the direct fired heater can achieve during certification testing for the burners that have been utilized in these types of heaters for many years. OSHA limits the short term exposure limit (STEL) of NO2 to 5 ppm, which allows for only a 15 minute timeframe. This ANSI Standard Z83.4 for direct fired heaters limits the combustion products to protect the health of the occupants within the space. It does not in any way limit the emissions based on their impact on the environment as it relates to compounds that contribute to the creation of smog and greenhouse gases. Nitrogen oxides (NOx) are recognized as a major contributor to both smog and greenhouse gases. Although the NOx emission for the existing technology of burner design is far from what may be considered as “Low NOx”, the operating efficiency of the direct fired heater lessens the impact of the total annual production of NOx compared to gas appliances that have lower operating efficiencies with lower NOx emission levels. Typically, the emission level of the existing burner technology has run between 55 ppm and 65 ppm at 3% oxygen (O2).
The South Coast Air Quality Management District (SCAQMD) has long been recognized as the leader in establishing the greenhouse gas emissions limits for gas utilization equipment by providing Rules for each type of appliance. Their primary focus has traditionally been on limiting the annual output of nitrogen oxides (NOx). By establishing NOx limits on an appliance category, this organization pushes the envelope of innovation by the equipment manufacturers. Their Rules apply to the localized areas around Los Angeles encompassing five adjoining counties. The 35 other Air Quality Management Districts in California continuously monitor SCAQMD activities and the results, and soon follow the lead of SCAQMD.
When SCAQMD first addressed large boilers, SCAQMD established a Rule for NOx emissions at 30 ppm at 3% O2. After manufacturers discovered technology improvement that consistently lowered their NOx emission results, SCAQMD revised the Rule to lower the allowable limit to 20 ppm at 3% O2 which continued to drive innovation. Eventually, these technological improvements led to the NOx emissions being lowered to 9 ppm at 3% O2 which is considered as ultra-low NOx. SCAQMD has focused on the largest sources of greenhouse gas emission in their efforts to reduce the smog in the immediate area around Los Angeles. As the air quality continued to improve, SCAQMD gradually shifted their focus to the smaller NOx generation sources.